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Thai rescuers on Thursday said they may be prodded into a complex extraction of 12 boys and their football coach from a flooded cave if forecast rain hammers the mountainside and jeopardises the rescue mission.

Thirteen sets of diving equipment have been prepared for the team, who have endured 12 nights underground in the Tham Luang cave complex in northern Thailand, a saga that has transfixed a nation and united Thais in prayers for their safe return.

Water is being pumped out from the deluged cave round-the-clock, reducing the flooding by one centimetre an hour.

But with rain forecast to begin on Friday, the Chiang Rai provincial governor heading the rescue effort conceded the mission was now "a race against the water".

"Our biggest concern is the weather. We are calculating how much time we have if it rains, how many hours and days," Narongsak Osottanakorn said.

In a sign of increased urgency, Mr Narongsak said medics and Thai Navy Seal divers were assessing whether the boys are fit and well enough to be taken out early.

But the prospect of the boys diving out is fraught with risk. It takes seasoned cave-diving experts about six hours to reach the muddy ledge where the boys are sheltering about four kilometres into the cave. Many of the youngsters – who are aged between 11-16 – are unable to swim and none have diving experience.

Rescue teams enter the Tham Luang cave complex in Thailand, where 12 boys and their football coach are trapped. AP

Three days after contact was made with the group, Thai Navy Seal experts continued to teach them the basics of diving.

But the areas where diving is still necessary are narrow and may require the boys to swim through murky waters unaccompanied.

In a two-pronged strategy, rescuers are also hunting for a chimney down to the boys, creating a second option for evacuation in the event that heavy rains force their hand.

They have enlisted the help of bird's-nest collectors from southern Thailand who are experts at finding hidden holes on forested cliffsides.

Authorities still hope they can manage any fresh deluge, with high-powered pumps draining 128 million litres of water so far from the cave in a round-the-clock effort.

"We are draining as much as we can," said Khao Khieupakdi, a Bangkok disaster-prevention official, who like scores of other specialists has been seconded to northern Thailand.

Bird's-nest collectors are escorted by park rangers as they search for openings into the Tham Luang cave complex. AFP

Water has been cleared from the entrance to a rescue base camp in "chamber three" inside the cave, but onward sections towards the boys remain impassable without diving, he said.

Officials have considered the possibility that the boys may have to remain in the cave until the monsoon season passes in three or four months.

But that option is a last resort and may have to be taken off the table if flooding worsens.

Concerns for the mental and physical health of the boys are also mounting after a prolonged ordeal in the dark, claustrophobic cave complex.

Experts say the risk of psychological damage is high for youngsters trapped in traumatic conditions, while the lack of light may cause confusion.

British cave divers found the emaciated group on Monday, huddled on a muddy shelf with flood waters lapping ominously below, after nine days missing.